Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is ejected in a continuous stream under pressure through at least one orifice or nozzle. The stream of ink is periodically perturbed by pressure regulation in accordance with digital data signals, causing it to break up into droplets at a fixed distance from the nozzle. At the break-up point, the droplets are charged and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from a nozzle directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Drop-on-demand systems are simpler than the continuous stream systems since they do not require ink recovery, charging, or deflection. There are three types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major component an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and the physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerance for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which employ piezoelectric devices to eject the ink droplets also suffer the disadvantage of a slow printing speed.
The second type of drop-on-demand system is known as acoustic ink jet system which expels ink through a nozzle or orifice by an acoustic method. Digital data signals are sent to the acoustic transducers located near the bottom of an ink reservoir and cause the formation of an acoustic wave which propagates through the ink. The acoustic wave is focused near the top of the ink level and provides the necessary energy to expel the ink out of the nozzle toward the recording medium which is located on the top of nozzle. With this type of acoustic ink jet device it is difficult to have multiple arrays of acoustic transducers and nozzles closely packed at a small distance with great precision. This ink jet system is not entirely suitable for high speed printing.
Another type of drop-on-demand printing system is thermal ink jet printing. In existing thermal ink jet printing systems (see U.S. Pat. No. 4,463,359), the printhead comprises one or more ink filled channels having one end communicating with a relatively small ink supply chamber or manifold, and having an opening at the opposite end referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels, at a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink in the immediate vicinity of the resistors with an instantaneously rise of pressure and form a bubble which expels an ink droplet. As the bubble grows, the ink experiences a pressure increase due to the evaporation of ink that bulges from the nozzle and is momentarily contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink in the back channel and the ink still in the channel between the nozzle and bubble start to move toward the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper and transparency. The depleting ink is refilled from the back channel which is connected to the ink supply system. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. Because the droplet of ink is emitted only when the resistor is actuated by digital data signals, this general type of thermal ink-jet printing is known as "drop-on-demand" printing. The thermal ink jet printing is also commonly known as "bubble-jet" printing. This type provides a simpler and lower cost device than the continuous stream, and yet has substantially the same high speed printing capability.
The printhead of U.S. Pat. No. 4,463,359 has one or more ink filled channels which replenish ink from an ink reservoir by capillary action. A meniscus is formed at each nozzle partially due to a small negative back pressure to prevent ink from weeping therefrom. The small negative pressure in the back (or back pressure) can be created by a capillary action or by placing the ink reservoir with an ink level at a position slightly lower than that in the ink channel. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth and collapse of the bubbles. The current pulses to the heater are properly applied to prevent excessive ink expulsion and premature breakage of the meniscus which can cause ink to recede too far into the channels after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet devices are known, such as those having linear and staggered linear arrays of printheads attached to the top and bottom of a heat sinking substrate and those having different color inks in different printheads for multiple color printing.
A common type of printhead is known as a "sideshooter." Sideshooters are so named because the ink droplets are emitted through the ink nozzle at a right angle relative to the direction of bubble formation and growth created by a heating element. U.S. Pat. No. 4,774,530 describes such a construction in greater detail. U.S. Pat. No. 4,638,337 discloses a sideshooter in which the sudden release of vaporized ink known as blowout is prevented by disposing the heater in a recess. Another type of printhead is known as a "roofshooter" which expels ink droplets from the nozzles in the same direction as that of bubble formation and growth.
In current practical embodiments of drop-on-demand thermal ink jet printers, it has been found that the printers work most effectively when the pressure of the ink in the printhead nozzle is kept within a predetermined range of gauge pressures. Specifically; at those times during operation in which an individual nozzle or an entire printhead is not actively emitting a droplet of ink, it is important that a certain negative pressure, or "back pressure" exist in each of the nozzles and, by extension, within the ink supply manifold of the printhead. A discussion of desirable ranges for back pressure in thermal ink-jet printing is given in the "Xerox Disclosure Journal," Vol. 16, No. 4, July/August 1991, p. 233. This back pressure is important for practical applications to prevent unintended leakage, or "weeping," of liquid ink out of the nozzles onto the recording medium surface. Such weeping will obviously have adverse results on print quality of a recording medium, as liquid ink leaks out of the printhead uncontrollably.
A typical end-user product in this art is a cartridge in the form of a prepackaged, usually disposable item comprising a sealed container holding a supply of ink and, operatively attached thereto, a printhead having a linear or matrix array of ink nozzles and channels. Generally the cartridge may include terminals to interface with the electronic control of the printer. Electronic parts in the cartridge itself are associated with the ink channels and nozzles in the printhead, such as the resistors and any electronic temperature sensors, as well as digital means for converting incoming signals for imagewise operation of the heaters. In one common design of printer, the cartridge is held with the printhead close to the recording medium or sheet on which an image is to be rendered, and is then moved across the recording medium or sheet periodically according to demand, in swaths, to form the image, much like a typewriter. Typically, cartridges are purchased as needed by the consumer and used either until the supply of ink is exhausted, or until the amount of ink in the cartridge becomes insufficient to deliver the ink to the printhead or until a blockage or clog occurs.
Other considerations are crucial for practical ink supply as well. The back pressure, for instance, must be maintained at a usable level for as long as possible while there is still a supply of ink in an ink cartridge. Therefore, a cartridge must be so designed and positioned as to maintain the desired back pressure within the usable range for as large a proportion of the total range of ink levels in the cartridge as possible. The back pressure can be provided by a capillary action of an ink storage medium or by adjusting the ink level of a reservoir relative to that in the printhead. Failure to maintain necessary back pressure causes the ink remaining in the cartridge to leak out through the nozzles of a printhead or otherwise be wasted.
In another design, the cartridge and printhead can be partitioned into several sections with different color inks and ink outlets which are connected to different ink inlets and channels of an ink jet printhead. Each color ink will have its own ink holding chamber or reservoir and ink outlet which is connected to its dedicated portion of the printhead comprising many ink nozzles and channels. This type of ink jet design allows printing of either a selected ink (e.g. black, cyan, magenta, yellow, etc.) or several color inks in a single swath mode. Color images can be produced on a recording medium or sheet as the printhead moves across it.
A fast ink jet printing method uses fullwidth arrays of abutted printheads including either linear or matrix arrays of nozzles. A fullwidth printing process employs a full-width array of printheads equipped with an array of heaters or resistors and ink nozzles. A fullwidth printing process includes the recording medium or sheet being moved at high speed past a linear array of nozzles which extend across the fullwidth of the printing zone of a recording medium. As soon as the linewise printing is carried out the recording medium is advanced to allow printing of the next line. Ink is usually supplied to the fullwidth array printhead from an ink reservoir.
U.S. Pat. No. 4,095,237 discloses an ink supply to a movable printhead in which a flow path is located in the flow path of a liquid reservoir of ink in communication with the printhead. The disclosed material for the filter is foam rubber or foam plastic. The printhead is raised higher than the outlet port of the reservoir.
U.S. Pat. No. 4,419,678 discloses a modular ink supply system for an ink printer wherein a liquid ink supply container is inserted into the printing apparatus, and communicating tubes puncture the container to form a tight seal against the outlet port and ventilation port of the container.
In earlier patents, felt substances have been used for the control of the flow of liquid ink. For example, U.S. Pat. No. 4,751,527 describes an ink jet "type printer" in which a plurality of holes are formed in a film and then filled with ink. Selectively heating areas of the film generate bubbles in the ink and eject the ink due to the pressure of the bubbles, thus printing an image on a sheet. In order to convey the ink to the film at the beginning of the process, felt ink supply members are employed to act as wicks for the gradual flow of ink into the film.
U.S. Pat. No. 4,394,669 discloses an ink jet recording apparatus having a printhead which moves relative to the copy surface. Felt members are employed to act as absorbing means to collect excess effluent liquid from the printhead.
U.S. Pat. No. 4,803,502 discloses an image formation cartridge having a number of rollers for applying ink to an image formation sheet. Each ink applying roller is in contact with an ink feeding element, which is made of a material such as polytetrafluoroethylene felt.
U.S. Pat. No. 4,771,295 discloses an ink-supply cartridge construction having multiple ink storage compartments. Ink is stored in a medium of reticulated polyurethane foam of controlled porosity and capillarity. The medium empties ink into ink pipes, which are provided with wire stainless filters for filtering of air bubbles and solid particles from the ink. The foam is also compressed to reduce the pore size therein, thereby reducing the foam thickness while increasing its density; in this way, the capillary force of the foam may be increased but at an expense of slower ink flow rate. The pore sizes of polyurethane are usually not uniform and they are difficult to control in the manufacturing process. Furthermore, additives, lubricants, and unreacted materials such as diisocyanates can interact with ink causing undesired dye absorption, pigment agglomeration; and ink contamination which can lead to poor copy image quality.
U.S. Pat. No. 4,791,438 discloses an ink jet pen (ink supply) including a primary ink reservoir and a secondary ink reservoir, with a capillary member forming an ink flow path between them. This capillary member draws ink from the primary reservoir toward the secondary ink reservoir by capillary action as temperature and pressure within the primary reservoir increases. Conversely, when temperature and pressure in the housing decreases, the ink is drawn back toward the primary reservoir.
U.S. Pat. No. 4,929,969 discloses an ink supply reservoir for drop-on-demand ink jet printing, including a medium in the form of a mass of foam material. This foam material comprises a three dimensionally branched network of fine filaments creating interstitial pores of uniform size. In preferred embodiments of the invention described, this foam material is a thermoset melamine condensate. In this patent, it is further pointed out that foam materials, when used as a medium for liquid ink, exert a controlled capillary back pressure. The melamine foam is somewhat brittle and can be easily broken during a fabrication process. The debris can get into ink channels in the printhead causing missing jets, exploding jets, ink misdirectionarity, and other problems resulting in poor image quality. Furthermore, the melamine formaldehyde foam in the ink cartridge is not chemically resistant. It can be partially attacked or dissolved by water and other ink ingredients at a temperature of about 50.degree. C., which can be reached during storage and shipment in hot weather. The dissolved foam material can deposit in ink channels of a printhead causing the blockage of ink paths and other printing problems.
Pending U.S. patent application Ser. No. 07/885,704, having the same assignee and which is incorporated herein by reference, discloses a system for supplying liquid ink to a thermal ink jet printing apparatus with a housing defining a single chamber having a ventilation port and an outlet port. An ink medium occupies at least a portion of the chamber, and is adapted to retain a quantity of liquid ink. A scavenger member, preferably made of acoustic melamine foam, is disposed across the outlet port providing a capillary force greater than that of the medium. A single layer filter can be attached to the scavenger.
The existing ink delivery systems fail to provide and maintain a high quality print with good optical density, in large part, due to the break-up and deterioration of the existing foam and felt ink mediums. The dislodged fibers particles and debris are identified as a large cause of ink channel blocking. Ink channel blockage can result in ink dropout, missing jets, exploding jets and other jetting problems. Although wire mesh or single layer filters have been used between the ink medium and the nozzle to filter particles, these filters suffer from inefficient filtration and blockage because they filter particles, debris or fibers on a single plane. This filtration causes slow ink refill and air ingestion problems at the printhead resulting in slow print speed and poor ink jet print quality. What is needed is an ink delivery and filtration medium that is capable of filtering out various size particles, fibers or debris while maintaining a strong and steady ink flow to the nozzle so that a high quality printing with good optical density can be achieved and maintained.